Ttiis research focuses on the molecular mechanisms of neuromodulation. Neuromodulators are typically neuropeptides, or monoamines such as dopamine, noradrenaline, and serotonin. Defects in neuromodulatory pathways do not usually lead to death, but can cause mental disorders such as autism, depression, schizophrenia, and attention deficit and hyperactivity disorder, as well as eating disorders and drug addiction. Neuromodulators are released from dense-core vesicles (DCVs). Little is known about DCV biogenesis, transport, and release, in part because it has not been possible to biochemically purify proteins specific to DCVs as it has for synaptic vesicles. Instead, a genetic screen in the nematode C. elegans was performed and successfully identified a set of novel molecules that act in a dense-core vesicle trafficking pathway. These include the small GTPase RAB-2 and two novel effectors, RUND-1 and CCCP-1. These molecules physically interact and are colocalized at the trans-Golgi network where DCVs are generated. Loss of these molecules leads to defects in sorting DCV cargos. This proposal aims to identify additional molecules acting In DCV trafficking and determine their mechanism of action. Aim 3 will identify more molecules acting in the RAB-2 pathway using genetic and biochemical screens. These new molecules will be characterized for their effects on DCV trafficking, their physical and genetic interactions with known molecules, and their cellular localization.'Using similar methods. Aim 4 will identify and characterize new molecules acting in a pathway with HID-1, in parallel to the RAB-2 pathway to regulate DCV trafficking. This research directly relates to the mission of NIMH, in particular to the first objective of the NIMH Strategic Plan: to investigate the causes of mental disorders. Mental illness can be caused either by too little or too much of certain neuromodulators. Thus, a better understanding ofthe mechanisms by which neuromodulators are released could lead to the development of drugs that either increase or reduce release, to compensate for the defect. In particular, molecules such as the ones I have identified that are necessary for release would be good targets for drugs to treat disorders due to too much release. Additionally, as dense-core vesicle pathways are modulatory rather than essential for neurotransmission, humans with mutations in these pathways would be expected to be viable, but mentally ill. Thus, the new genes identified in my work are good candidates for genes linked to mental health disease in humans.